Tire pressure monitoring systems (TPMS) on vehicles are generally required in the U.S., with Europe and countries in Asia to follow. The legislation mandating the use of TPMS typically sets a pressure warning threshold level which is monitored by wheel-based units, or wheel modules, in direct TPMS. The wheel modules are mounted inside of each tire, such as on the rim, valve or in-tire, in order to periodically or continuously monitor the inflation pressure of the tire.
Each wheel module typically includes a pressure sensor, control logic such as a microcontroller, a power source such as a battery, and a radio frequency (RF) transmitter that communicates pressure readings from the pressure sensor to central TPMS receiver mounted elsewhere in the vehicle. Some wheel modules also comprise an acceleration sensor for determining when the vehicle is in motion in order to conserve battery life. TPMS wheel modules typically include a unique identification code in the RF frame so that the central TPMS receiver can identify one wheel module from another, as well as distinguish wheel modules of one vehicle from those of another when vehicles are close enough that signals from one may reach another.
The process of identifying which wheel module sent a particular signal, and therefore which tire may have low pressure, is called localization. When a low pressure situation is detected, drivers generally want to know which tire is low, rather than simply that one of the tires is low which often requires each to be checked in order to determine which tire actually needs attention. Effective and efficient localization is an on-going challenge in TPMS because tires are frequently rotated and sometimes changed out between summer and winter, altering their positions. Additionally, power constraints on the wheel modules make frequent communications and localization signal transmissions impractical.
Some TPMS localize by including a low frequency initiator in the wheel well near each tire. In use, the initiator triggers its corresponding wheel module for a pressure reading on-demand. While such TPMS can effectively localize readings from each tire, they are relatively expensive and complex.
Another example localization scheme takes advantage of the acceleration sensor in the TPMS. As previously mentioned, an acceleration sensor is often included in TPMS for motion sensing by measuring the centrifugal acceleration force in g's. Given the mounting of the TPMS in the wheel and the rotation of the wheel when in motion, the orientation of the sensor changes during each wheel revolution (e.g., right-side up, sideways, upside-down, sideways, right-side up, etc.). This causes a signal change of +/−1 g because of the effects of gravity, which while rotating over time results in a sinusoidal ripple on top of the centrifugal acceleration signal. This signal can be measured by low-g sensors, such as are used for pure motion detection. Low-g sensors, however, have limited dynamic range (e.g., <50 km/h), which makes them undesirable for this type of localization. High-g sensors, such as those suitable for ranges of 250 km/h or more, can be used instead, though the challenge of measuring the sinusoidal signal on top of the centrifugal acceleration signal remains. It may be possible to utilize a high-resolution analog-to-digital (ADC) converter and/or an analog high-pass filter before the ADC, but these configurations increase power consumption and can add cost to the TPMS and/or require additional die area, making them impractical.
Therefore, there is a need for improved localization techniques.